Tone and volume control means



May 29, 1945.-- w. s. BACHMAN 2,377,121

TONE AND VOLUME CONTROL MEANS Filed April*'(', 1944 F. iF. DET.

T AHPLIHERS W DJ '2 U ll] D 2 ID \0 ll] 2 O D O .J

lo 00 I000 msuusucv Inventor:

William S. Bachm n,

His Attorney.

atenterl Ma ra 1%45 a rare TONE AND "\VQLUMIE (CUN'KROL MEANS William S. Bachman, lFairlield, lConrL, assignor to General Electric Company, a corporation of New York 8 i'llaims.

My invention relates tone and volume control means for carrier wave receivers such as those employed for ordinary household radio reception.

in ordinary household radio receivers both voice and music are commonly reproduced and it is desirable that the volume of sound reproduced be variable over an extremely wide range varying gradually from zero sound level to sound of large volume. quency components be reproduced with increased accentuation with respect to the high frequency components as the volume is reduced to compensate for reduced sensitivity of the human ear to low frequencies at low volume.

An object of my invention is to provide improved means to vary the volume of sound reproduced while maintaining desired relation between the high and low frequency tones.

Another object of my invention is to provide. such improved volume control means in which the accentuation of low tones increases smoothly as the volume is reduced and without any abrupt or sudden change in the relative proportions of high andlow frequency currents reproduced at any volume level.

'Ihe novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims. My invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing in which Fig. i represents an embodiment of my invention and Fig. 2 represents certain characteristics pertaining to its operation.

Referring to Fig. 1 of the drawing I have represented therein a radio receiver having an anlt is also desirable that the low fresistance 3 and condenser 3 are connected. The anode of this device is connected through a source of operating potential i2 to ground. This amplifier'thus operates as the usual cathode follower amplifier having all of its load impedance connected between the cathode and ground and. the lower terminals of the resistance 3 and condenser 5. The. source of operating potential 52 is bypassed by the usual bypass condenser it so that the entire audio frequency voltage output appears on resistances ii and it. The control electrode 6 is connected to a point between resistances 5 and it through. a grid leak resistance 54.

Resistances 9 and. it are preferably of comparatively low value, resistance 9 being for example of the order of 2,000 ohms and resistance it of the order oi 27,000 ohms. The voltage on these resistances is supplied to the control electrode of an amplifier it through a blocking condenser it? and two variable resistances ill and it in series. the cathode of amplifier id being connected to ground through the usual cathode bias resistance it which is shunted "by a bypass condenser 22. The point between resistances ill and it is connected to ground through a resistance 23 and a capacitance at, and similarly the con r01 tonne l and a radio -frequency portion 2 com prising, for example, the usual radio-frequency amplifier, converter, intermediate frequency amplifier, and detector, the output of the latter of which is illustrated at 3 and 4. That is, the detector may comprise the usual diode detector to which currents of intermediate frequency are supplied for detection, and which are detected and produce a unidirectional potential across condenser l and resistance 3 having magnitude varying in accordance with the audio frequency voltages to be reproduced.

The Voltages on resistance 3 and condenser l are supplied through condenser 5 to the control electrode 6 of an amplifier I. This amplifier has a cathode 8 connected through resistances 9 and [0 to ground to which the lower terminals of reelectrode oi device it is connected to ground through a resistance 25 and a capacitance 2e,

Operating potential is supplied fromthe source it to the anode of amplifier i5 through the resistance 2T. I

Amplifier it operates to amplify the voltage across the capacitive path 25, 26 produced by current flowing in resistances l"! and i8 and to/supply the amplified voltage to further amplifiers 28, if desired, and thence to the sound reproducer, or loudspeaker it.

In the operation of the system, the volume of sound reproduced by the loudspeaker 29 is dependent upon the values of resistances ii and it, these resistances being arranged to be varied together, as indicated by the arrowstherethrough which are joined by the dotted line 32, from a low value corresponding to high volume to a high value corresponding to low volume. As this variation occurs capacitances 2d and 26 operate increasingly to accentuate the low frequency tones relative to the high frequency tones which are reproduced as the volume is reduced.

For example, let us assume that the resistances ll and I8 are varied to substantially zero value.

. Then the full voltage which appears on resistances 9 and I0 is impressed directly across the shunt paths 23, 24, and 25, 26 and are supplied between the grid and cathode of the amplifier l5 for amplification and reproduction by the loudspeaker 29.

Condensers 24 and 26, under this condition,

have substantially no effect, on the frequency.

characteristic of the system by reason of the resistances 23 and 25 and the low impedance between the cathode 8 and ground.

Resistances I 1 and I8 are of comparatively high value, as for example of 3.3 megohms each, so that when these resistances are adjusted for their maximum value their impedance is extremely high relative to the impedance between the cathode of device I and ground, and the amount of sound reproduced by the loudspeaker 29 is substantially nil. Particular values of 2,000 ohms and 27,000 ohms have been mentioned for resistances 9 and 10, but it will be understood that the impedance between the cathode and ground is much less than the sum of these resistances owing to the fact that these resistances are shunted by the cathode-to-anode space of the discharge device I.

The actual value of the impedance between the cathode of device I and ground is dependent upon the gm, or transconductanceof the discharge device and may be very low, as in the order of 500 ohms if the discharge device 1 be one of the type commercially designated 6J5.

As these'resistances l7 and I8 are reduced to increase the volume, currents flowing therethrough and through the respective capacitance paths 23, 24 and 25, 26 produce voltages across these latter paths, the lower frequency components of course producing the greater voltages. These paths are proportioned to accentuate the low frequency tones for minimum volume and to decrease this accentuation as the volume is increased by reducing the resistances l1 and I3.

With the values of resistances previously mentioned and with resistances 23 and of the order of 33,000 ohms the capacitances 2d and 25 may be of the order of .01 microfarad. In this way it has been found that the acentuation of the low frequencies may be gradually varied as the volume is varied without any abrupt change in the tone at any point. Also the proportions or values of the different constantsof the circuit may be so adjusted that this acentuation may be made closely to match the characteristics of the human ear to produce the most pleasing effect.

In Fig. 2, for example, I have represented by dotted lines A, B, C, and D certain characteristics of the average human ear. These are character istics, which are well known, having been published in many texts and publications, and which were determined by experiment upon the ears of many people. These curves express the relation between sounds of equal loudness and frequency at different volumes of sound in the range below 1000 cycles. The corresponding curves in the range above 1000 cycles are substantially parallel and therefore require no compensation such as is provided by my invention and are not here illustrated. Curve A applies for high volume of sound and curve D for low volume of sound equalling the threshold of hearing. Curves B and C apply for intermediate sound levels. and J represent the calculated relation between frequency and voltage output supplied to amplifier l5 for different values of the volume control resistances l1 and I8. It will be seen that the curve H produced by my invention, which corresponds to a volume level comfortable for many home radio listeners-practically coincides with the curve B corresponding to the characteristics of the human ear for such volume level. Sim- Curves E, F, G. H.

mation is still very close.

ilarly the curves F and G practically parallel the curve A at a higher volume level. While at lower levels the slopes of the curves I and J depart slightly from that of curves C and D, the approxi- Of course the curve J corresponds to sound equalling the threshold of hearing, which is too low for normal listening purposes, and any departure from the ideal charateristic D is of no great importance. For higher volumes the curves corresponding to my invention approach almost ideally the characteristics of the human ear.

While thedesired tone versus volume characteristics may most readily be obtained with the invention as illustrated in Fig. 1 employing two series resistances I1 and I8 and two capacitance paths 23, 24, and 25, 26, it will be understood that the invention may be employedin a somewhat simplified form omitting, for example, resistance l8 and if desired the capacitance path 25, 26. Of course some impairment of the operation may result, but the desired advantages may in a large measure be secured.

Commonly, in the past, the volume control in a radio receiver has been effected by varying a potentiometer connected across the detector load resistance. To effect the desired accentuation of low tones at low volume, a portion of this volume control is shunted by a so-called bass tone compensation networ This network ordinarily comprises a simple shunt path comprising resistance and capacitance in series. For best operation two such paths are employed connected across the difierent-portions of the volume control potentiometer. While quite satisfactory results have been secured by this arrangement, at low volumes the low tones are not sufficiently ac-' centuated to avoid the thinness characteristic of radio receivers having such means of bass accentuation. In addition, as the volume is varied by variation of the potentiometer contact past the points to which these capacitance paths are connected, an abrupt change in tone may be noticed. These undesired efiects are completely avoided by my invention, in which the low tones are adequately accentuated at low volumes to overcome the insensitivity of the human ear at low frequencies, giving the desired richness and fullness to the tones to be reproduced. At the same time this accentuation is gradually reduced as the volume is increased without any abruptness at any point in the increase of the volume.

Of course some current flows through path 25, 26 even with resistances I1 and I8 adjusted for their maximum values. With resistances 23 and 25 properly proportioned with respect to resistances H and i8 respectively, the attenuation under this condition may be so great that the sound reproduced at the loudspeaker, even in the presence of the normal maximum signal voltage on the cathode 8, is of the order of the residual noise in the absence of a received signal and while its presence may be detected it is not of sufncient intensity to be objectionable.

While I have mentioned the impedance between the cathode 8 and ground as desirably being low, this impedance may be high without materially affecting the shape of the curves of Fig. 2. In so far as this impedance does affect these curves it is at high volume. If this impedance is high. however. a smaller proportion of the available signal is transmitted to the amplifier l5. This is clear since this impedance acts with resistance 23 as a voltage divider across the signal source. Thus additional amplification may be required in both places.

the receiver. Preferably this impedance is made a small part of thereslstance 23 so that substantially the entire available signal voltage is transmitted to the amplifier when the resistances if and i8 are at zero value.

In designing the circuit it is necessary first to determine the maximum attenuation required.

This attenuation determines the ratio between the resistances l i and 23, and between resistances i8 and 25. For example, if the desired attenuation be 80 decibels and resistances ii and it each be 100 times greater than resistances 23 and 25 respectively, then each network ll, 23, .24 and i3, 25, 26 produces 40 decibels attenuation and the system has the desired 80 decibels total attenuation.

It is next desired that this attenuation be limited to high frequencies, as for example frequencies above 1000 cycles. Condensers 2d and 26 are therefore inserted having values such that their impedances become large relative to respective resistances Hand 25 at low frequencies. In

the embodiment described these condensers have reactance equal to the respective resistances 23 and 25 at 500 cycles.

Condensers 24 and 26 may be proportioned to produce the degree of bass accentuation most pleasing to the particular listener. If desired, additional condensers 24 and 26 may be connected in shunt with respective condensers 24 and 26 to reduce the bass accentuation. The effect of these additional condensers is to shift the curves of Fig. 3 bodily toward the lower frequencies. If desired, these condensers may be connected in circuit individually and in succession to vary the bass accentuation.

It is of course necessary that a resistive path be present between the-control electrode of device l5 and ground to maintain proper operating bias potential on this control electrode. To this end a resistance of suitable value may be connected between ground and the control electrode at any desired point in the circuit. In the drawing I have shown a resistance 35 in shunt with capacitor 24 to form such a resistive path. This resistance, however, may be in shunt with capacitor 26 or higher resistancemay be used in Such resistance maybe sufficiently high to have no appreciable effect on the frequency attenuation characteristic of the system.

The bass accentuation of the system may be limited at low frequencies by use of resistance such a that shown at 35 in parallel with condenser 2t, and if desired another resistance in parallel with condenser 26, each of value equal to the reactance of the respective capacitor at the frequency below which itis desired to limit the accentuation.

Sometimes it is desirable to provide means sharply to attenuate the very high frequencies, as those above 3000 cycles. My invention lends itself readily to such attenuation by reason ofsince various modifications both in the circuit arrangement and in the instrumentaiities employed may be made, and I contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

" What I claim as new and desire to secure by Letters Patent of the United States is:

1. The combination, in a tone and volume control for audio systems transmitting voltages representing voice or music,- of an impedance on which said voltages appear, a variable resistance connected across said impedance through a condenser and an additional impedance, an output circuit connected across said condenser and additional impedance, whereby said variable resistance is connected in series with said first impedancc and said output circuit, said resistance being variable from a low value for high volume to a value high relative to said additional impedance for low volume, and said condenser and additional impedance having values to accentuate to a desired degree voltages having low frequencies in said output circuit relative to voltages having high frequenciesas said resistance is reduced.

2. The combination, in a tone and volume control device for audio systems transmitting voltages representing voice or music, of an impedance across which said voltages appear, an output circuit connected across said impedance through two variable resistances in series, and a condenser connected from one side of said impedance to a point between said resistances, said resistances I being variable together from low values for high volume to high values for low volume, and said condenser being proportioned increasingly to accentuate currents of low frequency in said output circuit as said resistances are increased.

3. The combination, in a tone and volume control device for audio systems transmitting voltages representing voice or music, of an impedance across which said voltagesappear, and an output circuit connected across said impedance through two variable resistances in series, each resistance forming part of an individual capacitive path across said impedance, said resistances being variable. together from low values for high volume to high values for low volumeand said individual capacitive paths being proportioned to accentuate currents of low frequency in said output circuit as said resistances are increased.

4. The combination, in a tone and volume control device for audio systems transmitting voltages representing voice or music, of an impedance on which said voltages appear, a pair of resistances in shunt with said impedance, a load device connected across one of said resistances and the other resistance being variable from a high value for low volume to a low value for high volume, the ratio between the value of said resistances being chosen in accord with the attenuation desired, and a condenser in series with said one resistance having a value such that at frequencies above a desired predetermined value said attenuation is determined substantially by the ratio between said resistances and said attenuation reduces at lower frequencies by reason of increased i'eactance of said condenser.

5. In combination, a sound reproducing device, a source of voltage representing sound waves, a pair of resistances connected in series across said source, means to supply voltage from one of said resistances to said sound reproducing device for reproduction thereby. the other of said resistances being variable to vary the volume of sound reproduced thereby, and a condenser in. series with said first resistance of value to accentuate the low tones of the sound reproduced sumciently to compensate for reduced sensitivity of the human ear at low frequencies.

6. In combination, a sound wave amplifier having input electrodes and an output circuit including a. sound wave reproducer, a source of audio voltage to be reproduced, said source being connected between said electrodes through two variable resistances in series, a resistance between said electrodes, a second resistance between one of said electrodes and a point between said variable resistances, said last resistances each having a value related to a corresponding one of said variable resistances to produce desired attenuation when said variable resistances have their maximum value, and a pair of condensers, each connected in series with one of said last resistances and'having a value to reduce said attenuation at low frequencies sufiiciently to compensate for reduced sensitivity of the human ear to sound of low frequency.

7. In combination, a sound reproducing device, a. source of voltage representing soundwaves, a pair of resistances connected in series across said source, means to supply voltage from one of said resistances to said sound reproducing device for reproduction thereby, the otherof said resist- I ances being variable to vary the volume of sound reproduced thereby, and a condenser in series with said first resistance, said condenser having a value such that the attenuation produced at frequencies above one thousand cycles per second is substantially determined by the ratio of said resistances and the reactance of said condenser being equal to the value of said one resistance at a frequency of substantially five hundred cycles per second.

8. In combination, a sound reproducing device, a source of voltage representing sound waves, a pair of resistances connected in series across said source, means to supply voltage from one of said resistances to said sound reproducing device for reproduction thereby, the other of said resistances being variable to vary the volume of sound reproduced thereby, a condenser in series with said first resistance of value to accentuate the low tones of the sound reproduced sufliciently to compensate for reduced sensitivity of the human ear at low frequencies, and a condenser in shunt with said voltage supply means to constitute with said other of said resistances a filter to attenuate high frequencies in the audible range.

WILLIAM S. BACHMAN. 

